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    <title>Document</title>
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    <script>
        let num = 3;
        if(num > 5){
            document.write("num的值大");
        }else {
            document.write("num的小于或等于5");
        }
        document.write("<hr/>");
        let boo = false;
        if(boo){
            document.write("boo为true");
        }else{
            document.write("boo为false");
        }


        document.write("<hr/>");
        let str="";
        /***
         * if语句的判断条件可以为true或false
         * 也可以为0(false)或非0(true)
         * 还可以是null(false)或非null(true)
         */ 
        if(str){
            document.write("str不为null");
        }else{
            document.write("str为null");
        }
    </script>
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